Liquid crystal materials exhibit such characteristics that the dielectric constant measured parallel to the molecular axis is different from the dielectric constant measured vertical to the molecular axis. Therefore, it is easy to line up the molecules either parallel or vertical to the external electric field. A liquid crystal electro-optical device controls the amount of light transmitted or scattered, utilizing this anisotropy of dielectric constant. In this way, desired portions are made dark or light. Known liquid crystal materials include twisted nematic liquid crystals, super twisted nematic liquid crystals, ferroelectric liquid crystals, polymeric liquid crystals, and dispersion liquid crystals. It is known that liquid crystals respond to the external voltage not after an infinitely short period but after some time. The time is different among liquid crystal materials and intrinsic to each different liquid crystal material. For twisted nematic liquid crystals, the time is tens of milliseconds. For super twisted nematic liquid crystals, the time is hundreds of milliseconds. For ferroelectric liquid crystals, the time is hundreds of microseconds. For dispersion and polymeric liquid crystals, the time is tens of milliseconds.
Of the liquid crystal electro-optical devices using liquid crystals, the active-matrix type has offered the highest image quality. In the prior art active-matrix liquid crystal electro-optical device, thin film transistors (TFTs) are used as active elements for activating liquid crystal pixels. An amorphous or polycrystalline semiconductor is employed as each TFT. A TFT of either p-type or n-type is used for each one pixel. Generally, an n-channel TFT is connected in series with each pixel. FIG. 2 is a schematic equivalent circuit diagram of the prior art activematrix liquid crystal electro-optical device. The liquid crystal portion of one pixel is indicated by 22. An n-channel TFT 21 is connected in series with this liquid crystal portion 22. Such pixels are arranged in rows and columns. Generally, a very large number of pixels such as 840.times.480 or 1280.times.980 pixels are arranged. In this figure, only a matrix of 2.times.2 pixels is shown for simplicity. Signals are applied from peripheral circuits 28 and 27 to the pixels to selectively turn on and off the pixels. Where the switching characteristics of the TFTs are good, it is generally possible to obtain a high contrast from this liquid crystal electro-optical device.
However, in such a liquid crystal electro-optical device fabricated in practice, the output signal from each TFT, or the input voltage V.sub.LC 20 (hereinafter referred to as the liquid crystal potential) to the liquid crystal, often fails to go high when it should go high. Conversely, the voltage sometimes fails to go low when it should go low. This phenomenon occurs because the TFT acting as a switching device applying a signal to the pixel assumes asymmetric states when it is turned on and off.
The liquid crystal 22 is intrinsically insulative in operation. When the TFT is OFF, the liquid crystal potential V.sub.LC is in a floating condition. This liquid crystal 22 is a capacitor in terms of an equivalent circuit. The potential V.sub.LC is determined by the electric charge stored in this capacitor. This charge leaks when the resistance R.sub.LC 24 of the liquid crystal is small or when dust or ionic impurities are present in the liquid crystal. If a resistance R.sub.GS 25 is produced between the gate electrode and the input or output terminal of the TFT 21 because of pinholes created in the gate insulating film on the TFT 21, then the charge leaks from this location. As a result, the potential V.sub.LC 20 takes a halfway value. In a liquid crystal display comprising a panel having 200 thousand to 5 million pixels, as many TFTs exist. Therefore, the abovedescribed problem takes place. This makes it impossible to accomplish a high production yield.
Although it is easy for most liquid crystal materials to produce two states, i.e., ON and OFF states, it is difficult to yield intermediate states. In the past, a thin film transistor (TFT) has been formed at each pixel using a liquid crystal. Thus, a so-called active-matrix circuit is formed. A method of producing gray tones has been proposed. In particular, the voltage applied to the liquid crystal of the pixel is subtly adjusted by the use of the TFT to give rise to such gray tones. However, the characteristics of the TFTs of the pixels vary in the range of as large as about 10%. The tolerable range of voltages for making the liquid crystal produce gray levels between black and white is narrow. For example, in the case of twisted nematic liquid crystals that are typical liquid crystal materials, the tolerable voltage range is usually only 10% or so of the threshold voltage of the liquid crystal. Hence, it is quite difficult to accomplish a gray scale by controlling the voltages. This means that the liquid crystal display is quite disadvantageous in competing with the CRT display which is a generally adopted display unit.